How do I remove the "posted on (date)"and "posted in: (category)

You will have to locate the code in your themes files to remove that, where/how all depends on the theme. Normally date uses the function at https://codex.www.ads-software.com/Function_Reference/the_date and the category is probably https://codex.www.ads-software.com/Function_Reference/wp_get_post_categories so find those and remove them. Post lists are usually in archive.php, single posts are normally in single.php and pages are normally in page.php, but again this varies from theme to theme.

A lot of the public themes are “blog like”, normally you will need to get a specialized paid for theme or get one made yourself for something different.

Which template?
Please mention the template name , I can help you to remove the exact lines